The manner in which various thalamic nuclei project to the cerebral cortex of the cat and rat will be examined in this study. Although the cat and rat are widely used subjects in neurological research, this aspect of their normal structure requires more detailed investigation. The results of such a study are important for understanding the transfer of incoming sensory information at the thalamic level and the regulation of this inflow by the cerebral cortex. Data from such a study, then, become applicable to various neurophysiological and behavioral problems. Small lesions will be placed in the thalamus and the resultant degeneration of axons will be studied in the cortex in sections prepared with the Fink-Heimer I technique. Wherever possible the functional roles of thalamic neurons to be destroyed will be determined prior to lesioning by recording their responses to appropriate natural stimuli.